This invention relates to methods and apparatus for biasing a semiconductor region and more particularly, to a method and apparatus for improving electron collection efficiency at a semiconductor region to reduce junction leakage.
Integrated circuits (ICs) are typically fabricated on a semiconductor wafer. The wafer typically is cut to form multiple semiconductor substrates or "IC chips". Semiconductor devices are formed on the wafer. Although the label "semiconductor" is used, the devices are fabricated from various materials, including electrical conductors (e.g., aluminum, tungsten), electrical semiconductors (e.g., silicon) and insulators (e.g., silicon dioxide). The semiconductive silicon wafer is subjected to deposition, etching, planarizing and lithographic processes to achieve the many semiconductor devices.
In fabricating semiconductor devices, substrates typically are doped to form various n-type and p-type regions. One layout structure is a well. FIG. 1 shows a p-type substrate 10 having a triple well. A center p-well 16 is surrounded by an n-well 20, which in turn is surrounded by another p-well 18. An n-tub 24 conjoins the n-well 20 and is formed below the center p-well 16. The center p-well 16 is separated from the p-type substrate 10 by the n-tub 24. Typically, one or more other doped regions 26 are included in the center p-well 16 to form an array of devices.
During normal operation, the n-tub 24 is biased to improve performance of a device array in the p-well 16. Specifically, the biasing voltage sets up an energy potential well in the n-tub 24 which collects electrons. As electrons fall into the energy potential well, they go into a lower energy state. Such electron collection reduces leakage of electrons across the junction into the p-well 16. For an array of DRAM cells, for example, the electron collection allows for a longer refresh period. Accordingly, it is desired that electron collection at the n-tub 24 be efficient.
The wider and deeper the energy potential well created, the more electrons that are collected. The factors determining the energy potential well performance include the biasing voltage and the doping level of the n-tub. Conventionally, the biasing voltage is limited to the supply voltage level. The doping level typically is limited to keep the n-tub from spreading too close to the surface.
When forming the p-well 16 and n-tub 24, the p-substrate 10 typically is doped in an area which is to become the n-tub. The n-tub, thus, defines a separation between pre-existing p-type regions. The doping process is an implantation of atoms. Implantation power defines how deep into the substrate the atoms are injected. Dosage level defines the number of atoms being injected, which affects the width of the n-tub junction for a given diffusion time. Typically, the implantation power is limited (e.g., 3 MeV maximum power) and, correspondingly, a maximum depth is defined. To maximize electron collection efficiency, it would seem that very high doping levels would be desired. However, because the doping level affects the width of the n-tub junction, excessive doping would expand the n-tub junction too close to the surface. More particularly, the n-tub junction would expand too close to other n-type regions formed or to be formed in the center p-well. FIG. 2 shows such an example. Beyond a given n-tub 24' thickness, an adjacent p-well 16' separation gets too small. Specifically, the p-well 16' separating the n-tub 24' from a nearby n-type region 28 in the p-well 16' becomes less than a minimum spacing required for region isolation. For a separation in the p-well 16' region between n-tub 24' and n-type area 28 which is less than the threshold separation, the local area defines an undesired "parasitic" leakage path (in effect, an undesired npn device 30). In order to avoid such a leakage path, the dosage for the n-tub is typically scaled back during the design process to keep the n-tub thickness to an acceptable level. A disadvantage of scaling the dosage back, however, is that the n-tub is comparatively less effective in collecting electrons.
Accordingly, there is need for improving collection of electrons at an n-tub.